Research conducted in May 2013 by the Foundation for Defense of Democracies and Roubini Global Economics revealed the bank exploited a "golden loophole" in the U.S.-led financial sanctions regime designed to curb Iran's nuclear ambitions. Here's how it worked: The Turks exported some $13 billion of gold to Tehran directly, or through the UAE, between March 2012 and July 2013. In return, the Turks received Iranian natural gas and oil. But because sanctions prevented Iran from getting paid in dollars or euros, the Turks allowed Tehran to buy gold with their Turkish lira -- and that gold found its way back to Iranian coffers.

Easy. Iran got more gold. Turkey could say it was sending cash to private citizens, thus not violating sanctions.

It was all good until January 2013, when the Obama administration decided to close this "golden loophole".

Instead of immediately charging Halkbank, though, the U.S. government essentially allowed its gold trading activities to continue until July 2013, says FP.

There are a couple reasons why the U.S. took this tack — Turkey is an important ally when it comes to Syrian policy. Plus, as we know now, the U.S. has been working out a nuclear deal with Iran.